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Dark tea (DT) holds a rich cultural history in China and has gained sizeable consumers due to its unique flavor and potential health benefits. In this study, headspace solid-phase microextraction (HS-SPME) combined with gas chromatography-mass spectrometry (GC-MS), relative odor activity value (ROAV), and chemometrics approaches were used to detect and analyze aroma compounds differences among five dark teas from different geographical regions. The results revealed that the five DTs from different geographical regions differed in types, quantities, and relative concentrations of volatile compounds. A total of 1372 volatile compounds of were identified in the 56 DT samples by HS-SPME-GC-MS. Using ROAV and chemometrics approaches, based on ROAV>1 and VIP>1. Eighteen key aroma compounds can be used as potential indicators for DT classification, including dihydroactinidiolide, linalool, 1,2,3-trimethoxybenzene, geranyl acetone, 1,2,4-trimethoxybenzene, cedrol, 3,7-dimethyl-1,5,7-octatrien-3-ol, β-ionone, 4-ethyl-1,2-dimethoxybenzene, methyl salicylate, α-ionone, geraniol, linalool oxide I, linalool oxide II, 6-methyl-5-hepten-2-one, α-terpineol, 1,2,3-trimethoxy-5-methylbenzene, and 1,2-dimethoxybenzene. These compounds provide a certain theoretical basis for distinguishing the differences in five DTs from different geographical regions. This study provides a potential method for identifying the volatile substances in DTs and elucidating the differences in key aroma compounds. Abbreviations: DT, dark tea; FZT, Fuzhuan tea; LPT, Guangxi Liupao tea; QZT, Hubei Qingzhuan tea; TBT, Sichuan Tibetan tea; PET, Yunnan Pu-erh tea; ROAV, Relative odor activity value; OT, Odor threshold; HS-SPME, Headspace solid-phase microextraction; GC-MS, Gas chromatography-mass spectrometry; PCA, Principal components analysis; PLS-DA, Partial least squares-discriminant analysis; HCA, Hierarchical clustering analysis., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Aroma is one of the significant quality factors of dark tea (DT). However, for a single large-leaf tea variety, there are few studies analyzing the effect of pile-fermentation on the aroma quality of DT. The GC × GC-QTOFMS, electronic nose (E-nose) and GC-olfactometry (GC-O) techniques were employed to analysis the difference of tea products before and after pile-fermentation. A total of 149 volatile metabolites (VMs) were identified, with 92 VMs exhibiting differential characteristics. Among these, 31 VMs with OAV > 1.0 were found to be correlated with E-nose results (|r| > 0.8). Additionally, GC-O analysis validated seven major differential metabolites. Notably, naphthalene, 2-methylnaphthalene, and dibenzofuran were found to enhance the woody aroma, while (Z)-4-heptenal, 2-nonenal and 1-hexanol were associated with an increase in mushroom, fatty and sweet odors, respectively. Moreover, 1-octen-3-ol was linked to reducing pungent fishy smell. These findings could provide a certain theoretical basis for understanding the influence of pile-fermentation on the aroma quality of dark tea., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023. Published by Elsevier Ltd.)

Dark tea fermentation involves various fungi, but studies focusing on the mixed fermentation in tea remain limited. This study investigated the influences of single and mixed fermentation on the dynamical alterations of tea metabolites. The differential metabolites between unfermented and fermented teas were determined using untargeted metabolomics. Dynamical changes in metabolites were explored by temporal clustering analysis. Results indicated that Aspergillus cristatus (AC) at 15 days, Aspergillus neoniger (AN) at 15 days, and mixed fungi (MF) at 15 days had respectively 68, 128 and 135 differential metabolites, compared with unfermentation (UF) at 15 days. Most of metabolites in the AN or MF group showed a down-regulated trend in cluster 1 and 2, whereas most of metabolites in the AC group showed an up-regulated trend in cluster 3 to 6. The three key metabolic pathways mainly composed of flavonoids and lipids included flavone and flavonol biosynthesis, glycerophospholipid metabolism and flavonoid biosynthesis. Based on the dynamical changes and metabolic pathways of the differential metabolites, AN showed a predominant status in MF compared with AC. Together, this study will advance the understanding of dynamic changes in tea fermentation and provide valuable insights into the processing and quality control of dark tea., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Pu-erh tea is post-fermented sun-dried tea leaves of Camellia sinensis (Linn.) var. assamica (Masters) Kitamura plant, native to Yunnan, China. Pu-erh tea is a highly prized commodity with unique aroma and taste and multiple health effects. This review overviews the chemical constituents, tentative bioactive compounds and their mechanisms responsible for bioactivities of Pu-erh tea. The bioactivities include antioxidative, antimutagenic, antimicrobial, laxative and neuroprotective activities, and controlling or preventing hypercholesterolemia, hyperglycemia, obesity, diabetes, osteoporosis and Alzheimer's disease. Limited human trials hardly convince the claimed therapeutic efficiency of Pu-erh tea. Raw and ripened Pu-erh tea possess their respective sensory characteristics, chemical and microbial diversities. Chemical and biological differences between Pu-erh and other tea types could be explained by different extents of their respective processing-induced compound transformations. Undesirable heavy metals, mycotoxins and other biocontaminants detected in Pu-erh tea relate to growing conditions of tea plantations, processing and storage conditions. Chemistry- and nutrition-derived mechanisms for tea pricing are lacking. Decontamination strategies and future studies for quality improvement of Pu-erh tea are recommended., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Diabetes and its related metabolic disorders are worldwide public health issues. Many studies have shown that changes in the structure and composition of the intestinal flora are closely related to the host's physiological and pathological processes. In this study, we aim to explore the effect of Liubao tea (LBT) extract on hyperglycemic mice with metabolic disorders and intestinal flora dysbiosis and to further study its regulatory effect on insulin resistance and its potential regulatory mechanism. Our results show that LBT had a good hypoglycemic effect and could significantly alleviate the metabolic disorder evoked by hyperglycemia. The gut microbial sequencing showed that LBT treatment increased the diversity of intestinal flora, increased the abundance of beneficial bacteria, and reduced the abundance of harmful or conditional pathogenic bacteria, as well as significantly altered 39 of the top 50 OTUs with abundance. Besides, LBT could activate the PI3K-Akt-PPARs-GLUT2 cascade signaling pathway to improve metabolic disorders, thereby alleviating insulin resistance. These results suggest that LBT has excellent potential to become a natural functional food for the prevention of hyperglycemia and insulin resistance., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Dark teas are rich in secondary metabolites, such as phenolics and flavonoids, which have been suggested to be associated with their health benefits. In this study, the concentrations of tea polyphenols, tea pigments, catechins, flavonoids, alkaloid, and volatile components in 44 dark tea samples, including Pu-erh, Fuzhuan and Liubao teas, were systematically examined. Among the samples tested, Pu-erh tea contained the highest total flavonoid content (5.24±0.05%), followed by Liubao (4.45±0.61%) and Fuzhuan teas (3.33±0.23%). The tea polyphenols levels in the dark teas were approximately 10%, and no statistically significant differences (p>0.05) were found among the different types. Hexadecanoic acid was the most abundant aroma component in the dark teas, accounting for 15-20% of the total volatile oils. Moreover, the antioxidant activities of these dark teas were analyzed using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay, 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) diammonium salt (ABTS) assay, ferric reducing antioxidant power (FRAP) assay, and cellular antioxidant activity (CAA) assay (HepG2 cells). The fat metabolism modulation activities (FMMA) of the dark teas were tested using a high-throughput screening method (SMMC-7221 cells). The results indicated that the different dark teas had diverse antioxidant activities, and the variation in the activities was significant. Correlation analysis showed that there was a significant positive correlation between the levels of EGCG and antioxidant activities measured using the ABTS (r=0.916) and FRAP (r=0.853) assays, and the levels of total flavonoids and theabrownins correlated well with the values determined using the CAA (r=0.845 and 0.865, respectively) assay., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Liupao tea is a distinctive Chinese dark tea obtained by indigenous tea fermentation facilitated by the symbiotic association of bacteria and fungi. The composition of fungal community in 4 Liupao tea samples stored for several years under natural microbial fermentation was evaluated by MiSeq sequencing. Taxonomic analysis revealed 3 phyla, 6 families, 8 genera. The genera Eurotium and Aspergillus were dominant fungi in almost all the samples. A total of 85 strains found in 41 other tea samples were species of Eurotium. amstelodami, Eurotium. niveoglaucum, Eurotium. repens, Eurotium. rubrum, Eurotium. tonophilum and Eurotium. cristatum by culture-dependent method. Of these species, E. repens, E. rubrum and E. tonophilum have not been previously associated with Liupao tea. This report is the first to reveal fungal flora composition using Illumina-based sequencing and provide useful information for relevant studies on the isolation of Eurotium species in Liupao tea. The predominant molds are Eurotium species, and the comparison of fungal diversity in dark teas is worth considering. The taxonomic analysis of the microbial community would also aid the further study of functional genes and metabolic pathways of Liupao tea fermentation., (Copyright © 2017. Published by Elsevier Ltd.)